Obtaining dn/dc of your polymer
1. Make a solution of your polymer. If is has aromatic groups, use a concentration of about 10 mgs/mL. If it’s acrylate based, bump it up to 15 or so. Know the concentration precisely. Use the 10 mL burrette to add the THF. Make sure the solution is homogenous.
2. Make sure the system has been recently calibrated with a 99k PS standard.
3. You have to make 5 injections at various concentrations. I have it set up for the machine to do the dilutions. While the Agilent pump is going to inject various amounts, the Viscotek always thinks it’s getting 100 µL. So on the Agilent, set up the first run like normal. For the 2nd one, use the method 80dndc. This injects 80 µL instead of the normal 100. Use 60dndc for the 3rd, 40dndc for 4th, and 20dndc for 5th. In the Viscotek software, set up five runs like normal. Make sure the injection volume is 100 µL on all runs. Also, name the files according to the dilution, IE, JRS-Poly-100, JRS-Poly-80, etc. Calculate the concentration for each sample. The first one should be the starting ~10mgs/mL. The injection using the 80dndc method will be .8*(original conc.). Fill the rest of the chart out. Then run all five samples.
4. Once done, set the baseline and put arrows around your peak for all five files. If the RI signal maxed out, don’t use that point.
5. Open one of your files, click on tools, trend view. A new window will pop up. Open up all five runs within that file. Then click method, open, and click on a method. Once done, click on method, calculate dn/dc. A chart will pop up with your line of RI area vs concentration. It should be linear, and the dn/dc is in the right hand corner. Don’t save it to the method unless you have created a new method.
6. If you want to create the chart yourself, you’ll need the RI of the solvent (THF = 1.405), the concentrations of your samples, and the RI area from each run, which you can get when analyzing your samples in the window that has the Mw, Mn, PDI, etc. You will also need to RI calibration factor. To get that open your run and apply a triple detection method. Click on method, edit method, and look under the calibration tab. The factor is right there. So enter your data into excel, plot RI area vs conc. Get the slope of that line with the regression software. dn/dc = (slope*1000*RI of solvent)/(RI calib factor*inj vol). The injection volume is 0.1 if you are using 100 µL. All of this can be found within the Viscotek help files, which is where I found it, so if you have any problems refer to that.